The object of the invention is to reduce or avoid the occurrence of these ensuing problems.
The solution according to the invention lies in the features broadly disclosed herein. Advantageous developments of the invention are the subject matter of the detailed disclosure.
An implant set comprises several reinforcing implants for insertion into the lamina of the vertebra, which reinforcing implants each comprise a main body with bearing surfaces on the vertebra and a fastening device, wherein according to the invention the main body has a front surface, a rear surface, and medial and lateral side surfaces, wherein the medial side surface is offset rearwardly in relation to the lateral side surface, and the medial and lateral side surfaces are designed to bear on sectioned surfaces of the lamina, and wherein a rearwardly protruding extension with a lateral bearing surface on the spinous process is arranged on the rear surface, and wherein the reinforcing implants of the set differ from one another in terms of the distance between medial side surface and lateral side surface.
The invention is based on the concept of making available a plurality of block-like reinforcing implants which differ in terms of their thickness, so as to be able to fill and reliably close resection openings of different widths in the lamina. The fastening device ensures that the reinforcing implant is fixed securely in its position in the lamina. By virtue of the reinforcing implant, the vertebral arch interrupted by the resection opening is completely closed again. Not only does this provide better protection for the nerve fibers running in the hollow channel, the mechanical stability of the vertebral arch is also restored and recovers the original values at the very latest when the implant has become incorporated.
The invention in this respect exploits the fact that, in the unilateral resection that is performed particularly often in practice (this is understood to mean access through the lamina only on one side, that is to say either to the left or right of the spinous process), sufficiently extensive and mechanically stable fastening to the spinous process is possible, and this possibility is made use of to fasten the reinforcing implant according to the invention. The claimed shape with the mutually offset side surfaces, which bear on the resected surfaces of the lamina after the resection, ensures a geometrically favorable integration of the reinforcing element in the vertebral arch, specifically in such a way that the reinforcing implant does not cause problems by protruding into the hollow space for the nerve fibers and also does not extend substantially outward. In other words, the main body of the reinforcing implant remains substantially inside the area that was filled by the corresponding part of the lamina prior to the resection. The danger of undesired irritation of the nerve fibers inside the hollow channel and also of the surrounding tissue is thus effectively countered.
A further advantage of the reinforcing implant according to the invention is that, after the correct reinforcing implant has been chosen from the set, only this part has to be inserted, and no other assembly work or adjustment work is needed deep within the operating site. It suffices to insert the implant of appropriate size and to fix it at the intended location by means of the fastening device, in the simplest case a bone screw. This ease of implantation thus safeguards against incorrect implantation and thereby contributes directly to improved outcomes.
Although the side surfaces are in most cases parallel to each other, they can form a wedge shape tapering slightly toward the front, the wedge angle measuring between 0 and 20°. The wedge angle is preferably less than 10°, more preferably less than 5°.
By virtue of the mutually offset arrangement of the side surfaces, the front surface and rear surface are oblique with respect to the two side surfaces, and they are in fact preferably substantially parallel or deviate from this by a maximum of 20°.
Advantageously, the side surfaces are not only mutually offset toward the front and rear, but also upward and downward. The main body thus expediently has a rhomboidal shape in two different planes. The rhombus angle (smaller internal angle) is here preferably between 35 and 75° in the vertical with respect to the underside and 30 to 60° in a plane orthogonal to the rear surface.
The lateral surface arranged further forward preferably forms a rounded apex angle with the front surface. This facilitates the insertion of the main body into the opening created by the resection, since the rounding prevents the implant from catching on the lamina in the event of an uneven shape of the resected surfaces, and the acute angle facilitates insertion, if appropriate with slight elastic widening (to achieve what is called a press fit), to the full thickness.
In order to avoid the reinforcing implant being inserted too far into the hollow space, a shoulder-shaped projection is preferably formed on one side surface and functions as an abutment. This ensures that a secure fit of the implant at the intended location can be achieved even without close visual monitoring and, in particular, it avoids a situation where the implant is pushed in too far and exerts pressure on the nerve fibers located in the hollow space. The danger of operating errors is thus effectively countered. It has proven useful that the shoulder-shaped projection is located at a distance from the front surface corresponding to approximately 0.8 to 2.2 times the thickness of the main body. It has proven useful to have a linear relationship with offset, such that, starting with a thickness of 3 mm, the distance is 6 mm, and the distance increases by 0.5 mm for each 1 mm of additional thickness.
A fastening hole is expediently provided on the rearwardly protruding extension. This not only provides securing by means of the lateral bearing surface and the force-fit thereof on the spinous process, it is also possible to achieve a form-fit fastening to the spinous process by insertion of a suitable fastening means (for example a screw). For this purpose, the fastening hole is preferably designed for the polyaxial reception of a screw. This is understood to mean that the screw with its head has a secure planar contact in the area of the fastening hole not only in an exactly central position, but also at angular deviations of up to 15° in each direction. In this way, even with a different anatomy of the vertebra, the screw can always be fitted in an orientation favorable for the fastening, preferably a translaminar screw. With this, a particularly secure hold can be achieved in the intact part of the lamina lying on the other side of the spinous process. However, a screw connection can also be provided directly on the spinous process; this is generally recommended when the opposite part of the lamina also has a defect. For this purpose, a screw dowel device is advantageously used. It permits secure fastening even in the case of a thin spinous process and in all situations in which, because of the small size of the fastening means used here, sufficiently reliable transfer of force would not be guaranteed by the screw thread alone. It comprises a dowel and a dowel screw. The dowel is preferably of sleeve-shaped configuration, with several segments which are connected at a near end and are free at their far end and have outwardly facing retainer hooks. They are dimensioned such that, when the dowel is pushed into the spinous process, they emerge on the other side and there engage behind the edge of the opening. The retainer hooks are preferably arranged in several steps with a height increasing toward the fixed end, in order to achieve a secure hold in spinous processes of different thicknesses.
The medial and lateral side surfaces are preferably provided with spikes. Proven shapes of the spikes are conical tips, pyramids, prismatic V-shaped elevations of different extent and height. They are advantageously configured asymmetrically, specifically in such a way that they have a greater bevel toward the front than in the opposite direction. This makes the implant easier to insert and provides a barb effect against undesired rearward migration. Secure primary fixation can be achieved in this way. In order to additionally increase the secondary fixation, the medial and lateral side surfaces are preferably provided with a coating that promotes bone growth. This can in particular be hydroxyapatite or other osteoinductive substances.
A laterally protruding fixing tongue is preferably provided on the rear edge of the lateral side surface. It is designed such that, in the implanted state, it rests on an outer surface of the so-called pars. In order to achieve good contact independently of the individual anatomy, the angle of the fixing tongue to the lateral side surface can preferably be changed. This can be achieved in practice, in a particularly expedient manner, by a flexible design of the fixing tongue, preferably with a reduced material thickness in the area of the transition between fixing tongue and main body. The fixing tongue can have a fastening hole, which advantageously has several defined receiving positions for a second fastening element, in particular a pars screw. The defined receiving positions make it possible to provide different positions for the pars screw in relation to the fixing tongue, wherein the pars screw is mounted with a form fit in each position, which is not the case in an oblong hole. As is also the case in the fastening of the translaminar screw, the receiving positions of the fixing tongue are preferably designed for polyaxial reception of a screw. In this way, the pars screw can be arranged not only with a translational degree of freedom but also with two rotational degrees of freedom in relation to the fixing tongue, which permits reliable fastening even in difficult anatomical situations. The angle range for the polyaxial receipt of the screw preferably once again measures approximately ±15° in each direction.
The reinforcing implant preferably has a tool receiver on its rear surface. This tool receiver allows the reinforcing implant to be received and held securely on a tool serving for the implantation. It is thus made easier for the operating surgeon to bring the reinforcing implant safely and precisely to the intended implantation site and to fasten it in place there. For this purpose, the tool receiver preferably has a longitudinal groove. This can be in one part or can be formed from several (also round) recesses. An unambiguous orientation of the reinforcing implant with respect to the tool can thus be achieved. A pulling thread is advantageously formed at the bottom of the tool receiver. It is thus possible, in combination with a holding screw on the tool, to secure the reinforcing implant on the tool and thereby not only to protect it against falling out and being lost, but also maintain it in a correct angular orientation.
A holding tool is preferably provided for this purpose, specifically such that it has a foot with an elongate gripper foot designed for interaction with the tool receiver. This gripper foot comprises, at the front end, a protruding area designed for complementary engagement in the longitudinal groove. It preferably comprises a connection of the gripper foot to a handle on a long hollow shaft through which a clamping element is guided that engages in the pulling thread. In this way, from the handle, the reinforcing implant can be clamped firmly on the gripper foot for safe implantation and, when the implantation site is reached, can be released therefrom, without the surgeon having to work deep within the operating site. It has proven useful in practice to arrange, at the rear end of the hollow shaft, a laterally extending projection, which has a predefined orientation with respect to the elongate gripper foot. The projection can preferably be a part of the grip. Thus, by taking hold of the instrument, it is already clear to the operating surgeon in which orientation the reinforcing implant clamped on the gripper foot is located. The danger of incorrect implantation as a result of incorrect orientation is thus reduced.
The material provided for the reinforcing implant is preferably a titanium alloy or pure titanium. This has the advantage of a high degree of biocompatibility in combination with good mechanical processing and load-bearing. Other materials that have proven useful are alloys of titanium/aluminum/vanadium, titanium/niobium/vanadium or cobalt/chromium/molybdenum, and also biocompatible plastics, such as polyether ether ketone (PEEK), or combinations of these materials.
The main bodies of the set according to the invention have different distances between medial side surface and lateral side surface (this distance is designated as the thickness). A range of between 3 and 15 mm has proven useful, and it has proven sufficient in practice to provide a gradation of in each case 2 mm. A millimeter gradation can also be provided for a finely graduated implant set.
The set according to the invention preferably additionally comprises mirror-inverted implants, which are likewise provided in different thicknesses. A treatment adapted to the anatomy can in this way be provided both in the left-hand area and also in the right-hand area of the lamina.